(1) Field of the Invention
The present invention relates to an improved process for forming homostructured mixed organic and inorganic cation exchanged layered silicates. In particular, the present invention relates to a process which produces protons and onium ions in galleries between silicate nanolayers by intercalating an onium ion precursor, such as an amine, into the galleries.
(2) Description of Related Art
Smectite clays are natural or synthetic layered alumino-silicates such as montmorillonite, bentonite, hectorite, fluorohectorite, saponite, beidellite, nontronite, and related analogs. Smectite clays have layered lattice structures in which the tactoids (crystallites) consist of stacked two-dimensional oxyanions separated by layers of hydrated cations. The oxygen atoms define layers approximately 10 .ANG.-thick, containing two sheets of tetrahedral sites and a central sheet of octahedral sites. The 2:1 relation between the tetrahedral and the octahedral sheets in a layer defines 2:1 layered silicates. For a typical 2:1 layered silicate, such as montmorillonite, the layer is made up of a central octahedral sheet, usually occupied by aluminum or magnesium, sandwiched between two sheets of tetrahedral silicon sites. Various isomorphous cation substitutions, e.g., Si.sup.4+ by Al.sup.3+ in the tetrahedral sheet, or Al.sup.3+ by Mg.sup.2+, or mg.sup.2+ by Li.sup.+ in the octahedral sheet, among others, also result in negatively charged nanolayers. These negatively charged layers are separated by hydrated cations such as alkali or alkaline earth metal ions in the gallery (interlayer) regions between the 2:1 layered silicates. The negative charge on the layer is balanced by interlayer of "gallery" cations, normally Ca.sup.2+ and Na.sup.+. The gallery cations in a natural smectite can be replaced by simple ion exchange process with almost any desired cation, including alkylammonium, alkyl phosphonium and other organic cations. Some idealized unit cell compositions and layer charge densities of smectite clays are listed in Table 1.
TABLE 1 Ideal Structural Formulas for some 2:1 Layered Silicates Layer Charge Density per Mineral Name Ideal Formula O.sub.20 unit Hectorite M.sub.x/n.sup.n+.yH.sub.2 O[Al.sub.6.0- 0.4-1.2 .sub.x Mg.sub.x ] (Si.sub.8.0)O.sub.2 0(OH).sub.4 Fluorohectorite M.sub.x/n.sup.n+.yH.sub.2 O[Al.sub.6.0- 0.4-1.2 .sub.x Mg.sub.x ] (Si.sub.8.0)O.sub.2 0(OH,F).sub.4 Montmorillonite M.sub.x/n.sup.n+.yH.sub.2 O[Mg.sub.6.0- 0.6-1.0 .sub.x Li.sub.x ] (Si.sub.8.0)O.sub.2 0(OH).sub.4 Nontronite M.sub.x/n.sup.n+.yH.sub.2 O[Fe.sub.4.0 ] (Si.sub.8.0- 0.6-1.0 .sub.x Al.sub.x)O.sub.2 0(OH).sub.4 Beidellite M.sub.x/n.sup.n+.yH.sub.2 O[Al.sub.4.0 ] (Si.sub.8.0- 0.8-1.0 .sub.x Al.sub.x)O.sub.2 0(OH).sub.4 Saponite M.sub.x/n.sup.n+.yH.sub.2 O[Mg.sub.4.0 ] (Si.sub.8.0- 0.6-1.2 .sub.x Al.sub.x)O.sub.2 0(OH).sub.4 Vermiculite Mg.sub.(x-z) y.sup.2+ [Mg.sub.6- 1.1-1.4 .sub.x Fe.sub.z.sup.III ] (Si.sub.8- .sub.x Al.sub.x)O.sub.2 0(OH).sub.4 Muscovite mica K.sub.2 [Al.sub.4.0 ] (Si.sub.6.0 Al.sub.2.0)O.sub.2 0 2.0 (OH).sub.4 Biotite mica K.sub.2 [Al.sub.y Mg.sup.6+.sub.(x/2)-(3y-2) ] 2.0 (Si.sub.6.0- .sub.x Al.sub.2.0+x)O.sub.2 0 (OH).sub.4 (x &lt; 1, y 21 2) Phlogopite mica K.sub.2 [Mg.sub.6.0 ] (Si.sub.6.0 Al.sub.2.0)O.sub.2 0 2.0 (OH).sub.4
Included in Table 1 for comparison purpose are the idealized compositions of 2:1 layered silicates, smectite clays, vermiculite, muscovite mica, biotite mica, and phlogopite mica. Vermiculite has a layer charge density higher than a smectite but lower than a mica. Micas usually have layer charge of 2.0. The gallery cations in a vermiculite or a mica can also be replaced by ion exchange, but the exchange processes are generally slower than for smectite clays. Also, vermiculites and micas exist commonly as single crystals that range in size from 10 .mu.m to 10 cm or larger. In contrast, smectite clays have sub-micron particle sizes. The particle size of vermiculite and mica can be reduced to the micron size range by mechanical grinding. Other ion exchangeable 2:1 layered silicate including illite, rectorite and synthetic derivative such as tetrasilicic mica and synthetic mica montmorillonite (SMM).
Those skilled in the art will know that smectites are members of a more universal class of layered inorganic ion exchangers. Many other layered inorganic cation exchanger materials can be selected in place of smectites. These layered materials include crystalline sheet silicate, layered phosphates, arsenates, sulfates, titanates and niobates.
The crystalline sheet silicates include kenyaite: Na.sub.2 Si.sub.20 O.sub.41.10H.sub.2 O; magadite: Na.sub.2 Si.sub.20 O.sub.41.3H.sub.2 O; makatite; Na.sub.2 Si.sub.4 O.sub.9.3H.sub.2 O; kanemite: NaHSi.sub.2 O.sub.5.3H.sub.2 O; revdite; Na.sub.2 Si.sub.2 O.sub.5.5H.sub.2 O; Grumantite: NaHSi.sub.2 O.sub.5.0.9H.sub.2 O; and Ilerite (octosilicate): Na.sub.2 Si.sub.8 O.sub.17.
The layered phosphates, arsenates, titanates and niobates are listed as follows:
TABLE 2 Class Compound general formula Phosphates H.sub.2 {M.sup.IV (PO.sub.4).sub.2 }.XH.sub.2 O, (M.sup.IV = Zr, Ti, Ge, Sn, Pb) CaPO.sub.4 R.H.sub.2 O (R = CH.sub.3, C.sub.2 H.sub.5), VOPO.sub.4.2H.sub.2 O, NbOPO.sub.4.3H.sub.2 O, H{SnCl(OH)PO.sub.4 }.2H.sub.2 O Arsenates H.sub.2 {M.sup.IV (AS.sub.4).sub.2 }.xH.sub.2 O, H{MnAsO.sub.4 }.H.sub.2 O (krautite), H{SnCl(OH)AsO.sub.4 }.2H.sub.2 O Titanates Na.sub.2 Ti.sub.3 O.sub.7,K.sub.2 Ti.sub.4 O.sub.0, Na.sub.4 Ti.sub.9 O.sub.20.xH.sub.2 O, K.sub.2 Ln.sub.2 Ti.sub.3 O.sub.10.H.sub.2 O Vanadates KV.sub.3 O.sub.8 Niobates KTINbO.sub.5,CSTi.sub.2 NbO.sub.7,A.sub.3 Ti.sub.5 NbO.sub.14, (A = Li, Na, K, Rb, Cs, Ti), KNb.sub.3 O.sub.8,K.sub.4 Nb.sub.6 O.sub.17, ACa.sub.2 Nb.sub.3 O.sub.10, (A = K, Rb, Cs) Molybdates MoO.sub.3 (OH),H.sub.x MoO.sub.3 Uranyl H{(UO.sub.2 PO.sub.4 }.4H.sub.2 O,H{UO.sub.2 AsO.sub.4 }.4H.sub.2 O Compound Manganates Busertite
Most important among the properties of smectite clays is the ability to replace the gallery cations in the pristine clay with almost any desired cations by ion exchange reactions. The exchange cations are very important in determining the ability of the gallery regions to imbibe (intercalate) neutral molecules. Inorganic cations (M.sup.n+) such as (Na.sup.+, Ca.sup.2+ and the like) prefer to be solvated by polar molecules such as water and certain polar organic molecules. Thus, these exchange forms are readily swollen by polar molecules, especially by water. Organic cations (alkylammonium, phosphonium ions and the like) are less hydrophilic, even hydrophobic, and prefer to intercalate organic molecules into the gallery regions. Inorganic cations such as K.sup.+ and Mg.sup.2+ in mica are anhydrous and strongly bound to the intergallery surfaces. Therefore, these silicates are difficult for gallery swelling and ion exchange reaction. However, the exchange of gallery cations in micas can be facilitated by reducing the particle size of the particles, preferably to average values of 2 .mu.m or less.
Clay-organic intercalates are intercalation compounds in which organic molecules enter the clay galleries and form definite compositions with specific clay basal spacings. The organic compounds that have been reported to form clay intercalates include uncharged polar organic compounds and positively charged organic ions, and ion-paired organic salts. These classes of guest species are intercalated by ion exchange, physical adsorption, or other mechanisms. Intercalation of organic polymers in clay minerals has been recognized to occur as natural processes in soils. Polymer adsorption and desorption also occurs under synthetic conditions (Theng, B. K. G. "The Chemistry of Clay-Organic Reactions", John Wiley & Sons, pages 136 to 206 (1974)). Interaction between clays and polymeric species has been discussed as natural or synthetic polymer adsorption and desorption (Theng, B. K. G. "Formation and Properties of Clay-Polymer Complexes". Elsevier pages 63 to 133 (1979)).
Mixed organic/inorganic ion exchanged forms of 2:1 layered silicates can potentially adopt one of several possible structures depending on the distribution of the distinguishable cations in the interlayer galleries. Organo cations particularly alkylammonium ions such as (CH.sub.3).sub.3 NH.sup.+ and (CH.sub.3).sub.4 N.sup.+ among others, are known to form interstratified structures when mixed with Na.sup.+ ions in the galleries of montmorillonite (Barrer, R. M. and K. Brummer, Trans. Faraday Soc. 59:959-968 (1963); and Theng, B. K. G., et al., Clay miner. 7:1-17 (1967)). In these interstratified phases the galleries are occupied by "onium-rich" and "sodium-rich" compositions of exchange cations. That is, the organic and inorganic exchange cations are "demixed" or largely segregated into separate galleries. Also, the stacking sequence of "onium-rich" and "sodium-rich" galleries is random in an interstratified mixed ion system. Interstratified onium ion/alkali metal ion smectite clays typically exhibit XRD spacings that increase with the amount of the larger onium ion occupying exchange positions in the galleries.
The segregation of organic onium ions and inorganic cations also has been recently observed for mixed ion exchange form of fluorohectorite containing equal molar amounts of a quaternary phosphonium ion, namely, (C.sub.18 H.sub.37)P(C.sub.4 H.sub.9).sub.3.sup.+, and an alkali metal ion, namely, Li.sup.+, Na.sup.+, or K.sup.+ (Ijdo et al, Advanced Materials 8:79-83 (1996)). In these latter compositions the organic and inorganic ions also are segregated into separate galleries, but unlike interstratified systems the stacking sequence of inorganic and organic galleries regularly alternates. This regular sequencing of galleries gives rise to ordered heterostructures that exhibit several orders of rational 001 reflections.
Yet another common behavior of mixed organic/inorganic exchange cation clays is the segregation of the two types of ions into homoionic tactoids containing long range stacking sequences of galleries that are occupied predominantly by one or the other cation. That is, the replacement of inorganic exchange ions by organic exchange ions occurs sequentially, gallery by adjacent gallery. Thus, if a fraction of the inorganic cations in a sample is replaced by organic ions, then one is left with a mixture of tactoids consisting of two homoionic end-member ion exchanged forms. These phase segregated mixed ion clays typically exhibit XRD powder patterns characteristic of a physical mixture of the homoionic, end-member forms of the parent organic and inorganic cation exchanged clays.
Randomly interstratified, heterostructured (regularly interstratified), and phase segregated mixed organic/inorganic ion clays and related 2:1 layered silicates have limited utility for commercial applications. The distributions of inorganic cations (I) and organic cations (O) in each of these three systems is schematically illustrated in FIGS. 1A, 1B and 1C, respectively. Because the organic exchange cations in each of these structures are largely segregated from the inorganic cations in separate organic-rich galleries, only those organic-rich galleries will be hydrophobic and suitable for intercalation and swelling by organic reagents and solvents.
It is the hydrophobicity of homoionic organic cation exchanged clays that makes them useful as materials for Theological control agents (e.g. in oil well drilling fluids, cosmetic formulations, and household cleaning products), adsorbents for toxic organic chemicals from water, and as components for organic polymer-layered silicate nanocomposite formation. Consequently, only the organic cation rich galleries of interstratified, heterostructured and phase-segregated mixed ion clays will be useful. The fraction of the clay containing inorganic-cation rich galleries will not participate in the desired intercalation chemistry with organic reagents, solvents and polymers. For this reason, fully exchanged homoionic organo clays, most typically quaternary ammonium ion clays, are used for the said applications.
One way of reducing the amount of expensive organic cations needed for hydrophobic intercalation and swelling is to mix organic and inorganic cations within the same galleries. Such mixed exchange cation forms may be said to be "homostructured" because each gallery in the tactoid would be compositionally equivalent and would exhibit uniform intercalation properties. The gallery distribution of inorganic ions (I) and organic ions (O) in a homostructured mixed ion intercalate is shown schematically in FIG. 1D. Homostructured organic/inorganic ion exchanged clays could, in principle, be made hydrophilic, hydrophobic, or even amphophilic depending on the relative population of organic or inorganic ions in the gallery. By adjusting the polarity of the galleries one can favor adsorption of guest species based on their intrinsic polarity. Also amphophilic galleries would allow co-adsorption of both organic and inorganic reagents for possible intragallery reaction. Still further, it should be possible using hydrophobic derivatives to adsorb organic reagents in galleries where the inorganic cation is an element capable of catalyzing reaction of the organic reagent (e.g. a transition metal ion). Despite the anticipated advantages of homostructured mixed ion clays, these structures are rare and very limited in the range of organic ion to inorganic exchange cation ratio. It has been suggested on thermodynamic grounds by Vansant and Uytterhoeven that homogeneous mixing of two exchange cations in every gallery should be possible. But such systems are very difficult to realize in practice. For instance, McBride and Mortland (McBride, M. B., et al., Clay Miner. 10:357 (1975)) observed random interstratification of ions in the replacement of Cu.sup.2+ ions by tetramethyl ammonium ions in montmorillonite. Also, Xu and Boyd (Xu, S., et al., Environ. Sci. Technol. 29:312-320 (1995); and Xu, S., et al., Soil Sci. Soc. Am. J. 58:1382-1391 (1994)) observed the segregation of (C.sub.16 H.sub.33)N(CH.sub.3).sub.3 and Ca.sup.2+ cations in the galleries of vermiculite. The exception are the Pinnavaia et al patents discussed below.
Demixed organic/inorganic ion exchanged forms of 2:1 layered silicates can adopt one of several possible structures that are distinguished on the basis of the distribution of the two types of cations in the interlayer galleries. Barrer and Brummer (Barrer, R. M. and K. Brummer, Trans. Faraday Soc. 59:959-968 (1963)) studied by X-ray diffraction the basal spacings and adsorption properties of a series of mixed CH.sub.3 NH.sub.3.sup.+, Na.sup.+ - and (CH.sub.3).sub.4 N.sup.+, Na.sup.+ -montmorillonites. The mixed ion compositions were prepared by ion exchange of Na.sup.+ -montmorillonaite with aqueous solution of the onium ion salt. They concluded that the compositions were "interstratified" structures. In these interstratified phases, the galleries are occupied by onium-rich and sodium-rich compositions of the exchange cations. That is, the organic and inorganic cations are largely segregated into separate galleries. Also, the stacking sequence of onium-rich and sodium-rich galleries is random. The structure of a randomly interstratified mixed ion 2:1 layered silicate is illustrated schematically in FIG. 1A.
Theng et al (Theng, B. K. G., et al., Clay Miner 7:1-17 (1967)) also have studied the replacement of Na.sup.+ and Ca.sup.2+ ions in montmorillonite by ion exchange reaction with alkylammonium ions in aqueous solution. They concluded, in agreement with Barrer and Brummer, that the products had interstratified, demixed structures.
Vansant and Uytterhoven (Vansant, E. F., et al., Clays Clay Miner 20:47-54 (1972)) studied by thermodynamic methods the partial replacement of Na.sup.+ by (CH.sub.3)NH.sub.3.sup.+, (C.sub.2 H.sub.5)NH.sub.3.sup.+ (C.sub.3 H.sub.7)NH.sub.3.sup.+ and (C.sub.4 H.sub.9)NH.sub.3.sup.+ onium in montmorillonite. They were inclined to interpret their results in terms of homogeneous mixtures of onium ions and Na.sup.+ ions in the galleries (i.e. in terms of a homostructure), but they believed that segregation of the ions into sodium-rich and onium-rich ions occurred upon drying the reaction products.
The homogeneous or uniform mixing of organic and inorganic cations in a smectite clay over an appreciable range of organic to inorganic cation ratios is very rare and limited to one very special reaction system in the known art. Thus, McBride and Mortland (McBride, M. B., et al., Clay Miner. 10:357 (1975)) observed that for the exchange of Cu.sup.2+ ions in montmorillonite by tetrapropylammonium ions, random interstratification of Cu.sup.2+ and tetrapropylammonium ions occurred up to 55% exchange. At 55% exchange, and beyond, the Cu.sup.2+ and tetrapropylammonium ions were distributed as in a homostructured mixed ion clay. This special behavior for homostructure formation, which is schematically illustrated in FIG. 1D, was attributed to the special ability of Cu.sup.2+ to reduce the degree of hydration by lowering the number of coordinated water molecules from six to four or less. Other inorganic cations do not show like reduction and stability in coordination number and do not form thermodynamically stable mixed ion clay homostructures.
In their studies of the replacement of alkali metal cations (Na.sup.+) and alkaline earth cations (Ca.sup.2+) in vermiculite by a long chain quaternary of the type used for forming organo clays (hexadecyltrimethylammonium, HDTMA.sup.+) Xu and Boyd (Xu, S., et al., Soil Sci. Soc. Am. J. 58:1382-1391 (1994) provided examples of "entrapped" mixed ion structures. At Na.sup.+ and Ca.sup.2+ concentrations of 0.005M and 0.001M, respectively, the inorganic cations became difficult to exchange after a certain mole fraction of (.about.0.6) of exchange sites were occupied by HDTMA.sup.+. The inability to displace all of the inorganic cations was attributable to an entrapment phenomenon that limited access to the inorganic exchange sites. Entrapment of the inorganic cation was caused by rapid edge collapse of the galleries around the organic onium ion. Greenland and Quirk, (Greenland, D. J., et al., Clays Clay Minerals 9:484-499 (1962)), observed that hexadecylpyridinium entrapped up to 25% of the Na.sup.+ in montmorillonite. Also, McBride and Mortland, (McBride, M. B., et al., Clays Clay Minerals 21:323-329 (1973)), observed that while tetrapropyl ammonium replaced .about.50% of the Ca.sup.2+ from montmorillonite, and only .about.10% of the inorganic ions were replaced from vermiculite, McAtee, (McAtee, J. L., J. C. American Mineralogist 44:1230-1236 (1959)) observed that long chain quaternary ammonium ions displaced most of the Na ions from montmorillonite, but entrapped a large fraction of Ca.sup.2+ at the exchange sites of the same mineral.
Inorganic cation entrapment by organic cations in 2:1 layered silicates can occur by several mechanisms that include a "covering-up" of the inorganic ion by the larger organic cation or a "contraction" of the gallery due to the presence of organic cation. Gallery contraction, however, is not a general mechanism because it requires a small organic cation capable of keying into the layered silicate surfaces to reduce the gallery height. Most onium ions expand the gallery relative to the size of the inorganic cation. Xu and Boyd have pointed out that both the "covering-up" and "gallery contraction" mechanisms are unlikely for onium ion with long alkyl chains. Instead, they favored entrapment. In this mechanism, replacement of the alkali metal or alkaline earth cation by the alkyl chains on the onium ions near the edges of the gallery create a hydrophobic barrier that impedes diffusion of the equated inorganic ions from the gallery. Thus, as illustrated in FIG. 1E, entrapped mixed organic-inorganic cation clays and related 2:1 layered silicates contain both types of ions within a given gallery, but in contrast to the homostructured forms illustrated in FIG. 1D, the ions of entrapped structures are not homogeneously distributed within the galleries and, therefore, are distinct.
Because the organic and inorganic ions are segregated within a gallery, that are entrapped mixed ion structures suffer the same disadvantages that are caused by demixing in phase segregated, interstratified and heterostructured mixed ion structures. However, as emphasized by Xu ad Boyd, entrapped structures are caused by hydrophobic and stearic factors and thus are metastable structures formed in a non-equilibrium exchange process. Phase segregated, interstratified, and heterostructured systems are thermodynamically stable phases formed in equilibrium exchange in an aqueous environment.
On the basis of the current state of the art, the mixing of organic and inorganic exchange cations in smectite clays and related 2:1 layered silicates is limited to compositions in which the fractions of the inorganic exchange cation (alkali metal alkaline earth metal or protons) represents less than 10% of the total cation exchange capacity. Moreover, ion mixing is further limited to onium ions with very short alkyl groups as in the mixed N(CH.sub.3).sub.4.sup.+ /Cu.sup.2+ and N(C.sub.3 H.sub.7).sub.4.sup.+ /Cu.sup.2+ systems of McBride and Mortland (McBride, M. B., et al., Clay Miner. 10:357 (1975)). Related work by Lee et al (Lee, J.-F., et al., J. Chem. Soc. Faraday Trans. I, 85:2953-2962 (1989)) postulates on the basis of surface area measurements that a small alkylammonium ion, namely N(CH.sub.3).sub.4.sup.+, will randomly displace Ca.sup.2+ ions on the gallery exchange sites of a smectite clay. But those skilled in the art will know that these homostructured mixed ion clays containing short alkyl groups would not be useful replacements for conventional, homoionic, long chain alkylammonium exchanged forms of smectite clays, because the sort chains alkyl onium ions would lack the hydrophobic character needed to cause gallery intercalation and swelling by organic agents.
U.S. Pat. Nos. 5,853,886 and 6,017,632 to Pinnavaia et al describes the preparation of proton exchanged clays and the use of these clays to form organic exchanged clays. The layered materials were particularly useful for preparing polymeric composites. U.S. Pat. Nos. 5,866,645 and 5,993,769 to Pinnavaia et al describe the preparation of onium ion and inorganic layered silicates and their use in the preparation of polymers.
Among the various applications of organo clays, their use as reinforcing agents, barrier components and Theological control agents for organic polymers is of great commercial value. In general, the polymer-clay composites can be divided into three categories: conventional composites, intercalated nanocomposites, and exfoliated nanocomposites. In a conventional composite, the clay tactoids exist in-their original state of aggregated layers with no intercalation of the polymer matrix between the layers of the clay. The polymer contacts the external surfaces of the clay particle (tactoids) exclusively. In an intercalated nanocomposite the insertion of polymer into the clay layer structure occurs in a crystallographically regular fashion, regardless of the clay-to-polymer ratio. An intercalated nanocomposite normally is interlayered by only a few molecular layers of polymer and the properties of the composite typically resemble those of the ceramic host (Kato, C., et al., Clays Clay Miner. 27 129 (1979); Sugahara, Y., et al., J. Ceram. Soc. Jpn. 100 413 (1992); Vaia, R. A., et al. Chem. Mater. 5 1694 (1993); and Messersmith, P. B., et al., Chem. Mater. 5 1064 (1993)). See also U.S. Pat. No. 4,683,259 to Goodman. In contrast, in an exfoliated nanocomposite, the individual 10.ANG. thick clay layers are separated in a continuous polymer matrix by average distances that depend on loading. Usually, the clay content of an exfoliated clay composite is much lower than that of an intercalated nanocomposite. Consequently, an exfoliated nanocomposite has a monolithic structure with properties related primarily to those of the starting polymer.
The exfoliation of smectite clays in a polymer matrix provides 10.ANG.-thick silicate layers with high inplane plane bond strength and aspect ratios comparable to those found for fiber-reinforced polymer composites. The clays used for nanocomposite formation are ion-exchange forms of smectite clays in which the Na.sup.+ and/or Ca.sup.2+ gallery cations of the pristine mineral have been replaced by organic onium ions. The onium ions may be protonated primary amines (RNH.sub.3.sup.+), secondary amines (R.sub.2 NR.sub.2.sup.+), or they may be tertiary amines (R.sub.3 NH.sup.+) or quaternary ammonium ions (R.sub.4 N).sup.+. The alkyl groups attached to nitrogen may be the same or different, and the alkyl groups may be replaced in part by a benzyl group (--CH.sub.2 --C.sub.6 H.sub.5), a phenyl group (--C.sub.6 H.sub.5) or by benzyl and phenyl groups. The alkyl groups may also be functionalized, as protonated .alpha., .epsilon.-amino acid with the general formula (H.sub.3 N--(CH.sub.2).sub.n --COOH).sup.+. Phosphonium ions may be used in place of ammonium ions for the formation of clay polymer nanocomposites.
Exfoliated clay nanocomposites formed between organo-cation exchanged montmorillonites and thermoplastic nylon-6 have recently been described (Fukushima, Y., et al., J. Inclusion Phenom. 5 473 (1987); Fukushima, Y., et al., Clay Miner. 23 27 (1988); and Usuki, A., et al., J. Mater. Res. 8 1179 (1993); and WO 93/04117 and 93/04118 describing thermoplastic polymers). Clay exfoliation in the nylon-6 matrix gave rise to greatly improved mechanical, thermal, and rheological properties, making possible new materials applications of this polymer (Usuki, A., et al., J. Mater. Res. 8 1179 (1993); and Kojima, Y., et al., J. Mater. Res. 8 1185 (1993)). Recently clay-reinforced epoxy nanocomposites have been reported (Lan, T. and Pinnavaia, T. J., Chem. Mater. 6 2216 (1994)) by using alkylammonium exchanged smectite clays in a flexible epoxy matrix, Lan. T. and T. J. Pinnavaia, Proceedings of ACS PMSE 71:527 (1994), Avelch et al, Clay Minerals 29:169-178 (1994) and Messersmith et al., Chem. Mater. 1719-1725 (1994)); U.S. Pat. No. 4,810,734 to Kawasuir describe various polymer exfoliated clays, PCT WO 95/14733 and PCT 96/08526 describe polymer exfoliated clays. The reinforcement of the exfoliated 10-.ANG.-thick clay layers was very significant. For instance, 15 wt % of the CH.sub.3 (CH.sub.2).sub.17 NH.sub.3.sup.+ -montmorillonite in the epoxy matrix increased the tensile strength 10 times and modulus 8 times. The significant reinforcing benefit provided by the silicate was especially significant for a flexible matrix. U.S. Pat. No. 4,889,885 to Usuki et al. shows thermoplastic vinyl polymer composites containing clay.
For all the polymer-clay nanocomposites reported to date, alkylammonium onium ions, or .alpha., .epsilon.-amino acid ions were exchanged into the clay galleries prior to nanocomposite formation, in part, to make the galleries more hydrophobic and better suited for interaction of polymer precursors. These organoclays allow intercalation (access) of monomer species (e.g., .epsilon.-caprolactone, epoxy resin and curing agent) into the clay gallery (Usuki, A., et al., J. Mater. Res. 8 1179 (1993), Lan, T. and Pinnavaia, T. J., Chem. Mater. 6 2216 (1994)). Upon polymerization reaction, the monomers form a network in the clay gallery regions and a polymer-clay nanocomposite is formed. By controlling the intra- and extragallery polymerization rate of the monomers, exfoliated and intercalated nanocomposites can be prepared.
Alkylammonium exchanged clays also have been used to form polymer-clay compositions by direct polymer melt intercalation (Vaia et al., Chem. Mater., 7 154 (1995)). The process involves heating a polymer-silicate mixture either statically or under shear in an extruder above the softening temperature of the polymer.
In the previous art, the presence of long chain alkyl onium ions in the clay galleries in place of essentially all of the alkali metal cations was essential for allowing the monomer or the pre-formed polymer to migate into the clay gallery. However, the alkylammonium ions in the gallery can block potentially favorable van der Waals interactions of the polymer matrix with the clay gallery surfaces. Also, the high cost of the alkylammonium ions and complex processing procedures limit the applications of the composites. Furthermore, the alkylammonium ions are toxic and require special handling procedures. Thus reducing the need for alkylammonium exchange cations in forming polymer-inorganic nanolayer composites would be a great practical and economical benefit.
Another problem restricting the use and performance properties of polymer-inorganic nanolayer hybrid composites is the difficulty in forming the composites with the inorganic nanolayers in the preferred exfoliated state. The prior art teaches two general ways of achieving inorganic nanolayer exfoliation in a polymer matrix. One approach is to form the polymer from monomeric polymer precursors or mixtures of polymer precursors in the presence of a layered inorganic ion exchanger interlayered by organic onium ions. However, in many cases the polymerization rate for polymer formation is much slower in the interlayer gallery region of the layered inorganic phase than in the bulk polymer. Consequently, intercalated rather than exfoliated hybrid nanocomposites are formed. Also, this "in situ" polymerization strategy lacks manufacturing versatility in the production of parts with hybrid nanocomposite compositions, because the nanocomposites can only be produced in batches of fixed polymer to inorganic nanolayer ratio.
The second approach to nanocomposite formation mixes a pre-formed thermoplastic polymer with the layered inorganic ion exchanger, typically modified with alkylammonium exchange ions. Melt processing the mixture under applied shear in an extruder can lead to nanocomposite formation under suitable circumstances. But melt processing is limited to thermoplastics with melting temperatures below the decomposition temperature of the onium exchange cation.
Organic-inorganic hybrid composites can exhibit mechanical properties superior to those of their separate components. To optimize the performance properties of these materials, it is usually desirable to disperse the inorganic components in the organic matrix on a nanometer length scale (Giannelis, E. P. JOM 4428 (1992); Gleiter, H. Adv. Mater. 4 474 (1992); and Novak, B. M., Adv. Mater. 5 422 (1993)). Smectite clays and other layered inorganic materials that can be broken down into nanoscale building blocks (Pinnavaia, T. J. Science 220 365 (1983)) are useful for the preparation of organic-inorganic nanocomposites.
U.S. Pat. No. 3,432,370 to Bash et al; U.S. Pat. No. 3,511,725 to Stevens et al, U.S. Pat. No. 3,847,726 to Becker et al and Canadian Patent. No. 1,004,859 to Nelson show various compositions incorporating epoxy resin. U.S. Pat. No. 5,552,469 to Beall et al also describes polymer exfoliated clays. There are numerous uses for these polymer matrices; however, there is a need to improve the properties of these polymers.
In view of the above limitations of the prior art, more versatile processing compositions and processing methods applicable to both thermoset and thermoplastic polymers are needed in order to more efficiently manufacture a broader range of polymer-inorganic nanolayer hybrid composite compositions.